The single country sim-cards are becoming more rare and most sim-cards work in other European countries you just have to pay slightly more with the 'roaming' charges. That is mostly for voice minutes but since you mentioned an iPad Mini you may find that prepaid data is surprisingly expensive and data roaming even on a European sim card could be pricy. It sounds like you have a 3g model and with that it may be worth talking to a provider on how much a data plan will be, they have gotten better lately. Otherwise the best use of the iPad in my opinion is as a wifi device and wifi will be at your hotel, cafes, some park, and many many other places. When on wifi you can use a lot of great apps and send email, etc. Here is more information for your planning.